Statistical Host Identification As a Test of Dark Energy

Project Information

SHADE

Grant agreement ID: 949572

DOI

10.3030/949572

EC signature date 10 August 2020

Start date 1 February 2021

End date 31 July 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 497 672,00

EU contribution

€ 1 497 672,00

Coordinated by

UNIVERSITY OF PORTSMOUTH HIGHER EDUCATION CORPORATION
United Kingdom

Periodic Reporting for period 3 - SHADE (Statistical Host Identification As a Test of Dark Energy)

Reporting period: 2023-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

Gravitational waves -- ripples in the fabric of spacetime itself -- were detected for the first time in 2015. This signalled the start of a new era of gravitational wave astronomy, which is profoundly different from the entire history of human astronomy to date (which is carried out using electromagnetic waves, i.e. light). Gravitational waves are produced by some of the most energetic and dramatic events in the universe, such as the merging of two black holes. They are inherently connected to the laws of gravity and the nature of black holes, and as such, have the potential to answer major outstanding fundamental questions in these areas.

The ERC Starting Grant SHADE is building crucial tools that allow the latest gravitational wave data to be brought to bear on these key scientific questions. Specifically, my team is constructing the statistical frameworks and software tools to deploy a new technique called `dark sirens', which maximally extracts information from gravitational wave detections. We are also creating new simulations of how the cosmic web of dark matter forms in the universe, to understand how this process is sensitive to the laws of gravity. The aims of SHADE are to create tools which go beyond the current frontiers in these areas, and then combine these with most recent data from both gravitational wave detectors and galaxy surveys. In this way, we seek to answer some of the biggest questions in cosmology: what are the true laws of gravity? What do these imply for the future expansion rate of the universe?

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

1) My team has led the development of a software package called gwcosmo, which is one of the main analysis tools used for cosmology by the LIGO-Virgo-KAGRA (LVK) collaboration (who carry out science with the current ground-based gravitational wave detectors). As such, our work is directly linked to some of the most high-impact results from the current gravitational wave (GW) data. In the current reporting period we re-designed and implemented a new statistical formulation of the dark sirens calculation performed by gwcosmo; this sped up the run time of the code by a factor of ~1000, allowing for more sophisticated analysis. We also extended the code to perform tests of the laws of gravity beyond the standard cosmological model. We tested our software on current public data and other mock data, and wrote up our work in two publications (both currently undergoing review for publication in the journal JCAP).

2) My team has produced a new simulation code to study the collapse of dark matter in the universe, called Hi-COLA. In particular, Hi-COLA can study the formation of the cosmic web within a very broad class of gravity theories beyond Einstein's General Relativity. Version one of Hi-COLA has been publicly released, and we presented the code and its initial results in a paper (published in JCAP).

3) We have developed a new algorithm for completing galaxy catalogues, i.e. modelling the missing galaxies from a survey. This is a key prescription that is used in the dark sirens analysis of 1).

4) We have performed a new set of calculations for relativistic effects in large-scale structure in luminosity distance space. These are a necessary step for future cross-correlations of gravitational wave data with galaxy surveys. This work also enabled us to model for the first time key properties of gravitational wave sources called evolution bias and magnification bias.

5) We studied how a change in the propagation speed of gravitational waves could be detected by both the future LISA space-based detector, and a `multi band' combination of future detectors. We find that a departure from GR can be strongly constrained via one or both of these methods, at lower energy scales than is possible currently. This will push our tests of General Relativity into a new regime, informing future ideas regarding dark energy (the mysterious substance which is thought to drive the expansion rate of the universe). This work is written up in two published papers.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The work carried out by by team is pushing beyond the state of the art in multiple ways. Firstly, the impressive speed-ups we were able to achieve in our dark sirens analysis package (gwcosmo) have enabled us to robustly include information about the mass distribution of compact objects. This is a key source of information which until a year ago could not be combined with the dark sirens method, due to computational speed issues -- this barrier is now removed. This development in turn enabled us to carry out tests of gravitational wave friction -- a kind of departure from GR -- which were fully robust, and combined information from black holes and neutron stars. Effectively, these are the most stringent constraints possible on this phenomenon at the moment.

Our simulation code Hi-COLA went beyond the state of the art by being the first code which is able to flexibly handle many different theories of modified gravity. Previously individual simulation suites were written and hard-coded for separate theories of gravity. Our work should hugely improve the efficiency of large-scale structure simulations in this regard, as well as opening up the study of many gravity theories which were not possible before.

Going forward, one key component of the grant work will be the analyse the latest data from the gravitational wave detectors -- this is being collected at present, and is not publicly available. We also hope to extend our analyses to incorporate a broader range of departures from GR. In Hi-COLA we plan to implement a new kind of screening phenomenon which will likewise generalise its scope. We will then be in an excellent position to start combining the results from the two project strands here, i.e. both gravitational and electromagnetic data. In this way we aim to make the most decisive statements about departures from General Relativity, having a significant impact on the community directions in both observational and theoretical cosmology.

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